1. Field of the Invention
The present invention relates to an optical luminescent element that is used together with an excitation light source such as a fluorescent lamp, and, more particularly, to an optical display device equipped with the optical luminous element which provides fluorescent light with controlled excitation fight.
2. Description of the Related Art
In the prior art optical device operative to absorb excitation light and to admit fluorescent light to come out thereof, a filter operative to transmit ultra-violet light and reflects fluorescent light has not been incorporated in a fluorescent lamp that emits fluorescent light when a fluorescent substance is excited by ultra-violet excitation light or a general light source system which causes a fluorescence luminous element to absorb excitation light other than ultra-violet light and to emit fluorescent light. In consequence, the optical element that is made so as to absorb fluorescent light traveling toward an excitation light source raises a decline in the output efficiency of fluorescent light.
In order to improve the output efficiency of fluorescent light from the fluorescence luminous element there have been proposed optical display devices equipped with a filter function in, for example, Japanese Unexamined Patent Publications Nos. 63-172120 and 9-159994.
Before describing the present invention in detail, reference is made to FIGS. 18 and 19 for the purpose of providing a brief background in connection with a prior art optical display device that will enhance understanding of the optical element and the optical display device equipped with the optical element of the present invention.
As schematically shown in FIG. 18, the optical display device 100A described in Japanese Unexamined Patent Publication No. 63-172120 comprises an ultra-violet light source 110a, a liquid crystal shutter 102, an interference filter 114 and fluorescence luminous elements 108a provided for three primary colors, red, green and blue, on the interference filter 114, which are arranged in this order. The interference filter 114 comprises a multi-layer (more than 20 layers) dielectric thin film that has an alternate structure of high refractivity dielectric layers and low refractivity dielectric layers or a multi-layer thin film that has an alternate structure of metal layers and dielectric layers. Such an interference filter 114 transmits ultra-violet rays L as excitation light for exciting the fluorescence luminous elements 108a and absorbs fluorescent rays M emanating backward from the fluorescence luminous elements 108a The interference filter 114 improves the resolusion and contrast of the optical display device 100A.
Further, as schematically shown in FIG. 19, the optical display device 100B described in Japanese Unexamined Patent Publication No. 9-159994 comprises an ultra-violet light source 110b, deflection plates 120 and 124 disposed on opposite sides of a liquid crystal light modulator 123, a reflection mirror 126 operative to reflect visible light M and fluorescence luminous elements 108b provided for three primary colors, red, green and blue, on the visible light reflection plate 126, which are arranged in this order. The visible light reflection mirror 126 comprises a multi-layer dielectric thin film like the interference filter 114 of the optical display device shown in FIG. 18 and operates to transmit ultra-violet rays L and to reflect forward scattered fluorescent rays M from the fluorescence luminous elements 108b. 
These prior art optical display devices described above by way of example increase the utilization efficiency of fluorescent light by reflecting the scattered fluorescent rays M traveling backward from the fluorescence luminous elements 108a or 108b by a multi-layer dielectric thin film, namely the interference filter 114 or the visible light reflection mirror 126. However, because the multi-layer dielectric thin film is formed by vacuum evaporation, in order for the multilayer dielectric thin-film to be capable of reflecting almost 100% of visible light incident thereupon, the multi-layer dielectric thin-film necessitates ordinarily consisting of several tens of layers. This results in high production costs of the optical display device.
It is therefore an object of the present invention to provide an optical element that is used together with a fluorescent light source and an optical display device that makes a display.
It is another object of the present invention to provide an optical element which employs an inexpensive filter other than an optical filter consisting of a multi-layer dielectric thin-film which is expensive.
It is still another object of the present invention to provide an optical element having a high utilization efficiency of excitation energy.
The foregoing objects of the present invention are accomplished by an optical element comprises a fluorescence luminous element operative to emit fluorescent light when excited by excitation light and a cholesteric filter comprising a cholesteric liquid crystal layer. The cholesteric liquid crystal layer as the cholesteric filter is formed over the fluorescence luminous element at one side of the fluorescence luminous element at which the excitation rays enter the fluorescence luminous element so as to transmit at least partly the excitation rays and to reflect at least partly the fluorescent rays traveling to the cholesteric filter.
The optical element transmits either right- or left-handed circularly polarized component of excitation light that is coincident in twist direction with a spiral structure of the cholesteric liquid crystal layer as the cholesteric filter and reflects a circularly polarized component of scattered fluorescent rays traveling to the cholesteric liquid crystal layer that is coincident in twist direction with the spiral structure of the cholesteric liquid crystal layer. In consequence, the optical element integrates the scattered fluorescent rays traveling backward to the cholesteric liquid crystal layer with the fluorescent rays directly coming out of the fluorescence luminous element, so as thereby to provide an increase in the utilization efficiency of fluorescent light.
The cholesteric filter may comprise two cholesteric liquid crystal layers having spiral structures opposite in twist direction, respectively, which are formed one on top of the other. This cholesteric filter can reflect both right- and left-handed circularly polarized components of scattered fluorescent rays.
The cholesteric filter may comprise a half wave plate element and two cholesteric liquid crystal layers having the same directional spiral structure between which the half wave plate is disposed. Because the half wave plate element reverses a circularly polarize component of excitation rays, the cholesteric filter reflects both right- and left-handed circularly polarized components of scattered fluorescent rays.
The cholesteric filter may further comprise a plurality of cholesteric liquid crystal layers operative to reflect visible light having wavelengths different from one another, specifically red, green and blue light, that are formed one on top of another. The optical element equipped with this type of cholesteric filter can reflect the entire range of visible light.
The optical element may comprise such a fluorescence luminance element as to emit visible fluorescent light or infrared light when excited by ultra-violet excitation light. In the case where the fluorescence luminance element is of a type which emits red, green and blue fluorescent light when excited by ultra-violet excitation light, the cholesteric filter is adapted so as to admit the ultra-violet excitation light to pass through and to reflect red, green and blue fluorescent light.
When the optical element is used to make a fulfill color display, the fluorescence luminance element is of a type which emits red and/or green fluorescent light when excited by blue excitation light. In this case, the cholesteric filter is adapted as to reflect red and/or green fluorescent rays traveling to the cholesteric filter and to transmit the blue excitation light The optical element thus structured can make a high brightness full color display.
The optical element may be united with a light source operative to emit excitation light such as a discharge lamp, an electro luminescence element and an electron-ray radiating element.
Further, the optical element can also be united with a light modulating element operative to modulate the fluorescent rays emanating from the fluorescence luminous element or excitation light from an excitation light source, such as a discharge lamp, an electro luminescence element and an electron-ray radiating element. When the optical element is incorporated in an optical display device, the scattered fluorescent rays traveling to and reflected by the cholesteric filter are modulated by the light modulating element. In consequence, the optical display device can convert the entire energy of excitation light into display light, which makes a display bright. The light modulating element may be disposed between the optical element and the excitation light source so as to modulate the excitation light from the excitation light source. This arrangement excites the fluorescence luminous element by excitation light after optical modulation, so as to cause the fluorescence luminous element to emit diffused fluorescent rays. In consequence, the optical display device can make a wide view angle of display without incorporating a diffusion element. In addition, because the cholesteric filter reflects scattered fluorescent rays traveling thereto, the optical display device provides an increase in the utilization efficiency of fluorescent rays. This is a contributory cause of a bright display.
The light modulating element may comprise any one of a liquid crystal element, an electromechanical light modulating element operative to modulate light due to electromechanical action and an electro-optical crystal. The electromechanical light modulating element may be of a type which changes a transmissible area that transmits light so as to control transmittance thereof or of a type which changes an optical length for interference so as to control transmittance thereof. The latter type of electromechanical light modulating element performs light modulation by changing an optical length for interference due to deformation of a flexible thin film, so as thereby to modulate near ultra-violet excitation rays for exciting the fluorescence luminous element. Further, electromechanical light modulating element may be of a type which performs total reflection of excitation rays incident thereupon at angles greater than a critical angle and admits proximity excitation rays (excitation rays that are affected by proximity effect) incident thereupon to pass through so as thereby to control transmittance thereof. This type of electromechanical light modulating element can perform high speed light modulation at a low driving voltage. In consequence, the electromechanical light modulating element is superior in the ability of displaying a moving object to the liquid crystal type of light modulator. Further, this electromechanical light modulating element provides an increase in the utilization efficiency of fluorescent light due to reflection of scattered fluorescent rays by the cholesteric filter. This causes the optical display device make a bright and high grade display.